It is well established that resistance to many diseases in plants is mediated by the interaction of plant genes referred to as “R” genes with corresponding Avr genes expressed by the pathogen (Hammond-Kosack and Jones (1997) Ann. Rev. Plant Physiol. Plant Mol. Biol. 48:1-39). This interaction leads to the activation of plant responses which in turn result in increased resistance to disease. This resistance is frequently mediated by the “hypersensitive response”, a localized cell death phenomenon that occurs in the areas of plant tissue invaded by the pathogen. This Avr/R interaction is race-specific; i.e., particular alleles of the plant R gene respond only to specific races of the pathogen which express a corresponding Avr gene (Flor (1971) Ann. Rev. Phytopathol. 9:275-296).
Most of the R genes characterized to date are dominant; i.e., resistance is exhibited by plants carrying the appropriate R allele in either a heterozygous or homozygous state. Some R genes have been isolated by map-based cloning. Many homologs of R genes have also been identified by sequence similarity. Because of the rapid evolution of plant resistance to disease, homology of a cDNA or genomic DNA sequence to a known R gene from a different plant species does not allow one to predict the pathogen specificity of that particular R gene. No general methods for such prediction exist today.
Genes involved in resistance of plants to plant pathogens may be used to engineer disease resistance into plants normally sensitive to disease using several different approaches. Transgenic plants containing alleles of R genes demonstrate resistance to corresponding races of the pathogen (Wang et al. (1996) Mol. Plan-Microbe Interact. 9:850-855). The resistance genes may also be engineered to respond to non-native signals derived from the pathogen. In addition, pathogen-derived Avr genes may be expressed in a controlled manner in plants to strengthen the response to a pathogen. Genes further downstream from the R genes in the signal transduction pathways that transmit signals to the disease response effector genes may be engineered to directly respond to pathogen infection, thereby shortening the response pathway.
The process of the hypersensitive response to a pathogen, and the signaling networks involved, are poorly understood. In no case have all of the genes involved in transmitting the signal from the pathogen to the site of the initiation of the hypersensitive response been identified.
In a few cases, the functioning of a disease resistance mechanism different from the Avr/R interaction described above results in a recessive, rather than dominant pattern of resistance. One such example is the Mlo gene of barley, which conveys resistance to Erysiphe graminis f. sp. hordei. The barley gene has been recently isolated by a positional cloning approach (Bueschges et al. (1997) Cell 88:695-705). The dominant (sensitive) allele (Mlo) is thought to encode a protein involved in regulation of leaf cell death and in the onset of pathogen defense. The partial or complete inactivation of Mlo results in the priming of the disease-resistance response even in the absence of the pathogen, and leads to increased resistance to E. graminis. 
The available scientific data concerning Mlo-mediated disease resistance in barley points towards another approach to controlling disease: priming the pathogen response pathway by diminishing the effectiveness of negative regulation of the hypersensitive response. Appropriately engineered plants may show increased pathogen resistance at the expense of expressing some pathogen response-related genes even in the absence of pathogen. Sense or antisense inhibition or targeted gene disruption of Mlo and Mlo-related genes may have such an effect. Resistance to other pathogens may also be increased using this approach.
Mlo-related cDNA clones and DNA segments of genomic DNA, and their homologs and derivatives, may also be used as molecular probes to track inheritance of corresponding loci in genetic crosses, and thus facilitate the plant breeding process. Moreover, these DNA sequences may also be used as probes to isolate, identify and genetically map Mlo and other closely related disease resistance genes.